Question: Is severe skin necrosis treatable without amputation of the flesh?

Skin necrosis occurs because of low blood and oxygen are supplied to a given skin region. Once necrosis has affected one area doctors cannot do anything for skin that has undergone the necrotic process. Skin necrosis can cause scarring. The earlier skin necrosis is detected the better it is as doctors can prevent further damage. The treatments used for skin necrosis are surgical debridement (removal of dead cells using scalpel) and antibiotic therapy.

Not yet. You either need to treat it before it becomes necrotic or removed the necrosed flesh.
Necrotic cells cause inflamation. They drive your immune system nuts, as it needs to remove the dead necrotic cells and prevent any further spread of the damage. The immune system can’t remove that many dead cells or get into the heart of them to kill an infection which can grow in there until it’s big enough ot be uncontrolled if it spreads (and kills you).
Removing the flesh (or limb in some cases) is a better outcome than being dead.